Automatic frequency control circuit



Sept; 27, 1949. J. c. SPINDLER 2,483,070

AUTOMATIC FREQUENCY CONTROL CIRCUIT I Filed May 2, 1946 s 2 DISCRIMINATORM W RECEIVER 2 7 SOURCE OF H VOLTAGE 17 DEFENDANT l m-Lo 0N AMBIENT ll TEMPERATURE l8 INVENI'OR.

BY Jase f! 062211148," 1

Patented Sept. 27, 1949 AUTOMATIC FREQUENCY CONTROL CIRCUIT J oseph: .C. SpindlenChicagmIll; assignor to- The Rauland. Corporation, Chicago; .Ill., a corporation oflllinois Application Mayi2; 1946, Serial NO.-6.66,554-

In :radio receivers which are selectively-tuned to transmissions on particular carrier frequencies it is frequently desirable to employ automatic. pparatus for controlling. the accuracy andmthe' constancy of tuning, In-.receivers.-ofthe super-- heterodyne type wherein tuning .is accomplished by varying theirequency-ofia local oscillatoreautomatio frequency control can.,.be-.accomplished by: automatic control.-of -.the. frequency; of the local oscillator This, .incturmcanhbe accomplished by automatic contro1., o the capacitance in the tank circuit of thellocal oscillator Circuits are known which. dependffor their operation onwhat is known as. a. variable reactance tube.v This typaofifcontrol.circuit does not directly employ either a .capacitor or an inductorwhose value can changein response to changes in. av control voltage. Instead; certain conventional components are arranged 'so that the circuit as a whole produces the .desiredeficti For certain purposes-much, for example", as to provide greater ease-ofrepairbecause its mode of operation is obvious and can lie-readily understoodby repairmen, the existence of a single circuitelement which'ds directlycapable, in itself; of .oifering. values of freactance which varyiin accordance with. changessin a .control'voltage has certain advantages.

Moreover, such asingle cir'cuit';e1ement might be small, compact,.inexpensive, sturdy, vdepend able,. and readily manufacturedflwh'ereas reactance .tube circuitsllack' these attributes. either partially or entirely.

It is anobject of .this invention to,,.provide automatic control .circuitshiOr varying the capacity of a capacitor in a predetermined'manner in response to controllvoltage changes;

It is a further. object of this invention to provide an automaticsfrequency control'circuit including such "an. automaticallyv variable capacitor connected to the tank.circuit of'the local oscil lator of a receiver .to be automatically tuned.

Other objects, features and advantages of this invention willlbecome apparent from thefollowing description .of this. invention taken .in connection with the drawings in which;

Fig. 1 is a circuit "diagram of one embodiment of this invention;

Fig; 2'is a schematic-representation of across section of the type of" variable" capacitor. employed in the :circuits 'of this invention; I and Fig; 3 isi'a :blockuiagramoftanotheryembodia ment of this invention? Referring more'particularly toiFig. 1, th portion' of the circuit comprising s'elements :I 2,--3 and.

his afilterxcircuitw Elements! and 2 are resistors; and elements 3 and 4-= are capacitors.

These-parts are arrangedso that capacitors-3- and 4- are in. shunt acrossethe; input-voltages fed to -this filter circuit whileiresistorsl and are m-series between -1 theinput and- 1 output =-ofthe filter. In thisresp cu-elements I, 2,3 and 4 are arrangedas a-conventional.resistor-capacitor alternating current filter.- In-the present circuit the filter is intended to eliminate audiecoma ponentswhich may reach the input and accord ingly thisportion' of the control circuit is'desigmated, for I all purposes inthis application, as the audio filter. Input terminalswfi and? a of .I the audio-filter are connectedacrossthe output terminals of a discriminator shown in. Fig.1 as block 5.-v When the device-is in.operation, th'e dise criminatorwill, at any instant of time, supply. voltages; between terminals 6 andfl which will include a-direct current component. Thiscomponent,. unless it. is of zero magnitude, will have somefinite positive or negative value.

Across the: output of'thisfilt'er therels connecteda gridresistorilf Grid-resistor Bis ground ed atone end. and its .other. end is ,connec-ted'to the grid of vacuum tubexywhich'mayibea triode. Th plate of vacuum tube l9 .is connected'to an energizingv source .of direct current voltage and: therefore, .as is well known, the current drawn by vacuum tube 9 will 'depend'on the voltage rela tionship between the gridand the cathode as well as .uponthe value of this energizing voltage. -The voltagerelationship between the grid and cathode oftubeQTde'pendS, in part; on the value of "resistorJUIthrQugh.which the cathode of vacuum tubev 9"is connected to ground. Theresistance ofjresistor I0 is" selected so that the voltage drop across it"rais'es the cathode of vacuum tube 9 above ground, potential and biases vacuum tube '9 at a biasvalue substantially midway between zero bias andcut-off' for the particular vacuum" tube used. As a result'of this. selectionof resistance and "of the energizing voltage; direct current voltage changes'which are impressed on the'grid of tube 8iwithin'the range of voltages oiierecl bythe'dis criminator. will causechanges in the current drawn'by'tube 9. The current through tube 9 will remain unidirectional but will change in magnitude whether these direct current voltage changes are". negative-going" or positive-going. These changes'in' thefcurrent'drawn by'tube'B will leadto corresponding changes inth'e current passing throughresistor 10 inasmuch as'this ele-' mentis' inseries Withtube' 9'.

Capacitor H is placed near enough to resistor ii! so that these two elements, for reasons explained hereinafter, will tend to have the same temperature.

Dielectric spacer I2 is located between the opposed plates of capacitor II. It is composed of a material whose dielectric constant varies in response to temperature changes. This property of spacer l2 causes the capacity of capacitor ll to be sensitive to temperature changes. Since this trait is to serve a useful purpose, herein, spacer [2, in particular, must be physically related to resistor it] so that it will tend to follow its variations in temperature. Some dielectric materials are available whose dielectric constants increase with rises in temperature whereas others are available whose dielectric constants decrease therewith. They are referred to respectively as having positive or negative temperature coefiicients of dielectric constant. It is obvious that a capacitor, of the kind described herein, which uses a spacer having a positive temperature coefficient will have its capacity increased by increases in its temperature. Such a capacitor is described as having a positive temperature coeflicient of capacity. Capacitors which are heat sensitive in the manner described herein and whose heat sensitive spacers are of a ceramic material are readily procurable and are used in precision instruments to compensate for ambient temperature change efiects.

The plates of capacitor II are connected into the tank circuit of local oscillator, block 13, of a superheterodyne receiver, block 14. In practice, discriminator as well as the entire control circuit shown in detail herein may be physically included in a complete receiver so that this automatic frequency control is an integral part of it. The function of the discriminator and its relationship to the RF portion of the receiver are conventional, and it is deemed unnecessary to describe it in detail herein. The term tank circuit as used herein and, in particular, in the claims, is intended to refer to the tunable resonant circuit of local oscillator l3 which, as will become apparent from the explanation which follows, is variably tuned by changes in the capacitance of capacitor l l.

Referring now more particularly to Fig. 2, convenient shapes for the plates of capacitor II and a convenient arrangement of these plates with respect to each other, to spacer l2, and to resistor 22 are shown. Capacitor H, as indicated therein, has concentric, substantially cylindrical plates with resistor in fitting within the inner plate and spacer l2 fitting concentrically between the inner and the outer plate. This entire assembly may be placed within a hollow conductive shield l5. The use of shield I5 is advisable since capacitor H is to be used as an oscillator tank circuit element. In this use its outer plate will carry high RF voltages. Shield IE will tend to avoid RF feedback to preceding and adjacent circuits. Shielding this same plate from resistor m, which is accomplished by the construction of the inner plate of capacitor l I shown in the drawing, also has a beneficial effect in that it tends to preserve the Q of the oscillator tank circuit.

Prior to placing this apparatus into operation, the local oscillator should be tuned and the receiver should be aligned while the direct current voltage on the grid of vacuum tube 9 is at zero value. This condition can be achieved by grounding the grid. By following this procedure, undesirable complications which might arise if the discriminator is misaligned can be avoided.

Preferably capacitor H should supply all of the capacitance needed in the tank circuit of the local oscillator. When, because of this, there is no other capacitor in the tank circuit and when capacitor H itself is not adjustable, the local oscillator may be tuned by varying the value of the inductance which is used in the tank circuit. This inductance may be varied by any conventional' means such as by varying the permeability of its core.

Thereafter, in operation, when the receiver which employs this automatic frequency control becomes detuned, a positive or negative direct current voltage, due to the action of the discriminator, will appear on the grid of tube 9. As indicated above, direct current changes within a predetermined range above or below zero will cause changes in the current flow through vacuum tube 9 and resistor 10. Resistor ID, in addition to being selected to efiect a proper bias of vacuum tube 9, is selected to have such characteristics that its temperature is appreciably altered by such changes in the current through it. Moreover dielectric spacer l2 of capacitor H, due to the proximity of capacitor H to resistor l0 and also due, in particular, to the design indicated herein, has these temperature changes quicldy communicated to it.

In this application the close physical association between resistor I0 and capacitor H which results in ready communication of heat changes from resistor ii! to spacer l2 will be referred to as thermal contact. 7

In fact, it is obvious that, through experimentation with the use of dielectric materials and of various designs, capacitors based on the broad operating principle of capacitor H might be devised which would have a wide variety of responses to changes in temperatures. While it is preferable for this control circuit that the capacitor be appreciably afiected by changes in temperature, that is, that its coefiicient of capacity be large, it is not essential that these changes necessarily be either in the direction of increases :or of decreases with particular respect either to increases or decreases in temperature. For, if, in a particular control circuit, it is found that changes in the value of capacity go in the opposite direction to that which is necessary to retune the local oscillator, the connections betwen the discriminator circuit and terminals 6 and "I need only be reversed to correct this condition.

Obviously, in the actual construction of frequency control circuits in accordance with this invention, the component elements should be selected to have values which will tend to cause the circuit to have maximum usefulness. For example, in a preferred embodiment, the capacitanceof capacitor H comprises-a, large percentage of all the capacitance of the tank circuit, the resonant frequency of which it is to control, and its temperature coefiicient of capacitance has a high value. For, when these conditions obtain, the device has broad usefulness in that it has a wide frequency range of control.

In said preferred embodiment, moreover, the portion of the circuit which includes resistor in and capacitor H is designed to operate at a steady state temperature which is relatively high with respect to anticipated ambient temperatures. When this condition obtains, this device is least aassgoro' affected by changes in ambient temperature. Obviously, however, the steady state operating temperature should be no higher than is consistent with good engineering practice, and allowance should be provided, in addition, for the greatest temperature rises which might be expected under extreme control conditions.

In said preferred embodiment, resist-or ID is arranged with respect to the inner plate of capacitor H and to spacer I-2 for ready heat transfer to them; provision is made for ready dissipation of heat from spacer l2 and the outer plate of capacitor II to the surrounding air; and all of these parts are selected to have small capacity for the retention of heat; that is, to have small thermal capacity. tain, this automatic frequency control circuit has a relatively fast corrective action and'a'minimum of thermal inertia.

Referring now more particularly to Fig. 3, block I6 represents a component having an electrical output which Varies in a predetermined manner with changes in ambient temperature. The output of'block Iii is connected directly across re.- sistor I0 and causes a certain normal or steady state electrical current at a certain predetermined normal or average ambient temperature, which current may either increase or decrease when the ambient temperature either drops or rises.

The plates of capacitor II are connected to terminals I1 and I 8 whereby capacitor II may be connected to any desired external circuit.

Thus for each change in the capacity of capacitor I I caused directly by a change in ambient temperature (heating or cooling of the unit by the surrounding air instead of by resistor I0) this circuit may provide a boost aiding the direct change. On the other hand due to the large transformation in the magnitudes of ambient temperature changes, as measured near to resistor III, which are caused by this device, the direct effects on the value of capacitor I I may be of a negligible order of magnitude. If they are negligible, it may be entirely feasible to devise block I6 so that it supplies a current which decreases with ambient temperature rises. Although the temperature of resistor I0 will then tend to be eifected in opposite directions directly by the surrounding air and indirectly by the circuit action, and though the two tendencies will be opposing rather than I aiding, yet one of them will be negligible. The direction of change in the capacity of capacitor II does not depend on whether block l6 provides an increasing or decreasing electrical flow for a given ambient temperature change. For, as explained above, spacer I2 may be selected to have either a positive or a negative temperature coefficient of dielectric constant. This increases the number of equivalent alternate arrangements and the number of different useful element combinations.

Obviously the over-all effective response of the value of capacitor I I to temperature changes may be closely controlled over a wide range in various ways and its effective ambient temperature coefiicient of capacitance may be precisely set at any desired value over a wide range. Available control parameters which suggest themselves are the value of the resistance of resistor 10, the rate of change of current flow for changes in ambient temperature, and the magnitude of the nominal or steady state current.

It is obvious that the embodiment of this invention shown in Fig. 3 may be used to increase the effectiveness of a compensating capacitor in When theseconditions o'b=- temperature compensated circuits where the temperature result in complete compensation.

In practice resistor I0 and capacitor I I, I2 will constitute a unitary structure comprising a single element. In this element heat changes occurring in the resistor portion will produce heat changes in the condenser portion and for the purpose of 1c the claims this relationship is defined as thermal cont'act'between the two portions.

What I claim is:

1. In an electrical circuit, an element comprising a resistor and a capacitor in thermal contact with each other, said capacitor having at least two opposing plates and dielectric spacing means, said spacing means being composed at least in part of material havin a dielectric constant the valueof which is affected by temperature changes, means for supplying electrical energy to said resistor including means for varying the amount of said electrical energy whenever the capacity of said capacitor ceases to be substantially that of apredetermined value, and an external circuit connected to plates of said capacitor.

2.- In an electrical circuit, an element comp-rising a resistor and a capacitor in thermal contact with each other, said capacitor having at least two opposing plates and dielectri spacing means, said spacing means being composed at least in part of material having a dielectric constant the value of which is affected by temperature changes, means for supplying electrical energy to said resistor and for varying the amount of said electrical energy in response to ambient temperature changes within a predetermined temperature range, and an external circuit connected to plates of said capacitor.

3. In an electrical circuit, an element comprising a resistor and a capacitor in thermal contact with each other, said capacitor having at least two opposing plates and dielectric spacing means, said spacing means being composed at least in part of material having a dielectric constant, the value of which is affected by temperature changes, a circuit tuned to a predetermined frequency, means for supplying to the tuned circuit voltages including a component at said predetermined frequency means for supplying electrical energy to said resistor, including means for varying the amount of said energy when any change occurs in the relationship of the frequency of said component to the tuning of said circuit, and an external circuit connected to plates of said capacitor.

4. In an electrical circuit, an electrical device having at least an anode, a cathode and a control grid, an input circuit for feeding a varying voltage to said control grid, means for energizing said electrical device, an element comprising a resistor and a capacitor in thermal contact with each other, said resistor being in series with said anode and said cathode, said capacitor having at least two plates and dielectric spacing means, said spacing means being composed of material whose dielectric constant is affected by temperature changes, and an external circuit connected to plates of said capacitor.

5. In an electrical circuit, an electrical device having at least an anode, a cathode and a control grid, an input circuit for feeding varying voltage to said control grid, means for energizing said electrical device, an element comprising a resistor and a capacitor in thermal contact with each other, said resistor connected between said cathode and ground, said capacitor having at least two plates and dielectric spacing means, said spacing H means composed of material having a temperature coeflicient of dielectric constant of relatively large value, and an external circuit connected to plates of said capacitor.

6. In a radio receiver, a, local oscillator, a frequency discriminator, an electrical device having at least an anode, a cathode, and a control grid, an input circuit for feeding varying voltage from said discriminator to said control grid, means for energizing said electrical device, an element comprising a resistor and a capacitor, said resistor being in series with the anode and cathode, said capacitor having at least two plates and dielectric spacing means composed at least in part of material the value of whose dielectric constant is affected by heat, said local oscillator having a tank circuit with which said capacitor is connected.

"I. In a radio receiver as in claim 6, said resistor being connected to said cathode and to ground.

8. In a radio receiver, a local oscillator, a frequency discriminator, an electrical device having at least an anode, a cathode and a, control grid, an RF filter circuit having its input terminals connected to the output of said discriminator and its output terminals between said control grid and said cathode, means for energizing said electrical device, an element comprising a resistor and a capacitor in thermal contact with each other, said resistor being in series with the anode and the cathode, said capacitor having at least two plates and dielectric spacing means being composed at least in part of material the value of whose dielectric constant is affected by heat, said local oscillator including a tank circuit with which said capacitor is connected.

9. In a radio receiver, according to claim 8, and in which said resistor is connected to said cathode and to ground.

JOSEPH C. SPINDLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,019,765 Osnos Nov. 5, 1935 2,093,331 Lynn Sept. 14, 1937 2,111,414 Work Mar. 15, 1938 2,114,846 Little Apr. 19, 1938 2,206,238 Rochow July 2, 1940 FOREIGN PATENTS Number Country Date 586,528 Germany Oct. 23, 1933 

